Subjective Type Questions: Analytical Chemistry- 2 | JEE Advanced | 35 Years Chapter wise Previous Year Solved Papers for JEE PDF Download

Q.12. Calcium burns in nitrogen to produce a white powder which dissolves in sufficient water to produce a gas (A) and an alkaline solution. The solution on exposure to air produces a thin solid layer of (B) on the surface. Identify the compounds A and B. (1996 - 2 Marks) 

Ans. Sol. The reactions are given as follows :

Q.13. A colourless inorganic salt (A) decomposes completely at about 250°C to give only two products, (B) and (C), leaving no residue. The oxide (C) is a liquid at room temperature and neutral to moist litmus paper while the gas (B) is a neutral oxide. White phosphorus burns in excess of (B) to produce a strong white dehydrating agent. Write balanced equations for the reactions involved in the above process. (1996 - 3 Marks) 

Ans. Sol. Let us summaries the given facts.

 Strong white dehydrating agent

(i) Since the resulting dehydrating agent is derived from P, it is likely to be P₄O₁₀.

(ii) P₄O₁₀ is produced by burning phosphorus in excess of neutral oxide (B) which is likely to be NO₂.
(iii) Thus the salt A should be NH₄NO₃ which explains all given reactions.

Q.14. During the qualitative analysis of a mixture containing Cu²⁺ and Zn²⁺ ions,  H₂S gas is passed through an acidified solution containing these ions in order to test Cu²⁺ alone.
 Explain briefly. (1998 -  2 Marks) 

Ans. Sol. The solubility products of CuS and ZnS are

Since K_sp (CuS) << K_sp (ZnS), very small concentration of S^2- is sufficient to cause the precipitation of Cu²⁺ ions. In order to have very small concentration of S^2- ions, acidic medium is used. Due to the common ion H⁺, the ionisation of H₂S is suppressed:

The available concentration of S^2- ions in acidic medium causes only the precipitation of CuS and not that of ZnS.

Q.15. A white solid is either Na₂O or Na₂O₂. A piece of red litmus paper turns white when it is dipped into a freshly made aqueous solution of the white solid. (1999 - 4 Marks) (i) Iden tify the substance and explain with balan ced equation. (ii) Explain what would happen to the red litmus if the white solid were the other compound. 

Ans. Sol. (i) The substance is Na₂O₂. When dissolved in water, the solution becomes alkaline with the liberation of H₂O₂ Na₂O²⁺ 2H₂O → 2NaOH + H₂O₂

NOTE : Due to the alkaline solution, the red litmus paper will turn into blue, which subsequently changes into white due to oxidation caused by H₂O₂. (ii) The substance Na₂O merely produces alkaline solution and thus the red litmus paper will turn into blue.
Na₂O + H₂O → 2NaOH

Q.16. An aqueous solution containing one mole of HgI₂ and two moles of NaI is orange in colour. On addition of excess NaI the solution becomes colouress. The orange colour reappears on subsequent addition of NaOCl. Explain with equation s. (1999 - 3 Marks)

Ans. Sol. Sodiumiodide on reaction with HgI₂ gives colour less complex salt, Na₂[HgI₄]

Colour is due to presence of residual HgI₂ But on addition of excess NaI, it becomes colourless due to change of residual HgI₂ into Na₂[HgI₄] HgI₂ + 2NaI (excess) → Na₂[HgI₄] (colourless) The orange colour of HgI₂ reappears due to conversion of Na₂[HgI₄] into HgI₂ by means of NaOCl

Q. 17. An aqueous blue coloured solution of a transition metal sulphate reacts with H2S in acidic medium to give a black precipitate A, which is insoluble in warm aqueous solution of KOH. The blue solution on treatment with KI in weakly acidic medium, turns yellow and produces a white precipitate B. Identify the transition metal ion. Write the chemical reactions involved in the formation of A and B. (2000 - 4 Marks) 

Ans. Sol. Summary of the given facts

The reaction corresponds to copper sulphate.

Q.18. Write the chemical reactions associated with the ‘borax bead test’ of cobalt (II) oxide. (2000 - 3 Marks)  

Ans. Sol. 

Q.19. A white substance (A) reacts with dilute H₂SO₄ to produce a colourless gas (B) and a colourless solution (C). The reaction between (B) and acidified K₂Cr₂O₇ solution produces a green solution and a slightly coloured precipitate (D). The substance (D) burns in air to produce a gas (E) which reacts with (B) to yield (D) and a colourless liquid.
 Anhydrous copper sulphate is turned blue on addition of this colourless liquid. Addition of aqueous NH₃ or NaOH to (C) produces first a precipitate, which dissolves in the excess of the respective reagent to produce a clear solution in each case. Identify (A), (B), (C), (D) and (E). Write the equations of the reactions involved. (2001 - 10 Marks)

Ans. Sol. 

The above set leads to following conclusions.
(i) Since the gas (B) is colourless and turns acidified K₂Cr₂O₇ solution green, it should be H2S.
(ii) Since H₂S gas is obtained by the reaction of dil. H₂SO₄ on A, the latter must be sulphide.
(iii) The white colour of the sulphide (A) poin ts out towards ZnS.
Thus the various reactions can be written as given below.

Q.20. When a wh ite crystallin e compound X is h eated with K₂Cr₂O₇ and concentrated H₂SO₄, a reddish brown gas A is evolved. On passing A into caustic soda solution, a yellow coloured solution of B is obtained. Neutralizing the solution B with acetic acid and on subsequent addition of lead acetate, a yellow precipitate C is obtained. When X is heated with NaOH solution, a colourless gas is evolved and on passing this gas into K₂HgI₄ solution, a reddish brown precipitate D is formed. Identify A, B, C, D and X. Write the equations of reactions involved. (2002 - 5 Marks) 

Ans. Sol. Let us summarise the given facts.

NOTE :  Reaction of compound X with NaOH solution and subsequent treatments indicate that X has NH⁴⁺ radical. On the other hand, reaction of X with K₂Cr₂O₇ solution, conc.
H₂SO₄ and subsequent treatments indicate that A has Cl^– radical. Thus compound X is NH₄Cl which explains all the above reactions.

Q.21. A mixture consists of A (yellow solid) and B (colourless solid) which gives lilac colour in flame.
 (a) Mixture gives black precipitate C on passing H₂S(g) through its aqueous solution.
 (b) C is soluble in aqua-regia and on evaporation of aquaregia and adding SnCl₂ gives greyish black precipitate D.
 The salt solution with NH4OH gives a brown precipitate.
 (i) Th e sodium car bonate extract of th e salt with CCl₄/FeCl₃ gives a violet layer.
 (ii) The sodium carbonate extract gives yellow precipitate with AgNO₃ solution which is insoluble in NH₃.
 Identify A and B, and the precipitates C and D. (2003 - 4 Marks)

Ans. Sol.